As a person who is quite fond of Froslass, I can give some of my input.
Being frail is actually a niche for Froslass. It can get off a Destiny Bond with very high Speed while it's down to its Focus Sash. To be honest, I've never used an Icy Wind set. While lowering the opponent's Speed is certainly good, in singles the lowered Speed doesn't matter much due to the heavy amount of switching. This is why I much prefer Ice Beam or Shadow Ball.
Anyway, onto the set itself. It's actually very good when used correctly, as I use it with Shadow Ball over Icy Wind (I have only used Froslass on a mono-ice team, so the Ghost move was pretty essential for extra coverage). Spikes is for setting entry hazards. Taunt + Focus Sash + Destiny Bond allows you to get almost always KO unless one of two things occur -- the opponent switches or the opponent is faster/has a priority move. The first one isn't exactly bad, since you can set a layer of Spikes. Destiny Bond is since you'll most likely be brought to your Sash. The last slot is for Taunt.
Now, why is it in UU?
First, for the UU viability thread, Froslass is in the B Rank.
As the other answer stated, it can set up Spikes safely due to being a spinblocker. This is very important, increasing its overall viability.
In UU, it's faster than a lot of Pokemon, allowing it to pull off its signature suicide lead strategy. Here are the Speed Tiers.
So, all in all, it has a great strategy that is very reliable in UU. It can shut down set up sweepers and walls, outspeed a good chunk of the metagame, and is one of the best entry hazard setter, in my opinion (because of its ability to spinblock and deny most Defog users with a fast Taunt).
Hope this helps! :)